[2/5] VSA: Collapse Gateway+Usecase into *Service per feature

[Mpdreamz]

Why

The ports-and-adapters split required three types per feature (interface, gateway impl, usecase orchestrator) where the usecase added only ~5 lines of logging and DTO mapping over a single implementation. This made every feature harder to navigate and contributed no reuse benefit since all three layers lived in the same project.

What

Each feature now has a single interface and implementation that owns the full request/response flow — logging, Elasticsearch calls, response mapping, and cursor encode/decode where applicable:

• IFullSearchService / FullSearchService (was IFullSearchGateway + FullSearchGateway + FullSearchUsecase)
• INavigationSearchService / NavigationSearchService
• IChangesService / ChangesService
• IOtlpService / AdotOtlpService
• IAskAiService / IAskAiMessageFeedbackService (renamed from Gateway variants)

AddElasticDocsApiUsecases is renamed to AddElasticDocsApiServices throughout. Interfaces are kept for test seams (mocks updated to MockSearchService).

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Part of a move to vertical slice architecture to be able to cleanly lift search contract:

1. [1/5] VSA: Merge Elastic.Documentation.Api.* into single project #3333 — Merge Elastic.Documentation.Api.* into single project
2. [2/5] VSA: Collapse Gateway+Usecase into *Service per feature #3335 — Collapse Gateway+Usecase into *Service per feature
3. [3/5] VSA: Slim Elastic.Documentation, extract heavy deps to Tooling #3345 — Slim Elastic.Documentation, extract heavy deps to Tooling
4. [4/5] VSA: Slim Configuration to YAML schema; organize root namespaces #3347 — Slim Configuration to YAML schema; organize root namespaces
5. [5/5] VSA: Extract Elastic.Documentation.Search.Contract as zero-dep library #3350 — Extract Elastic.Documentation.Search.Contract as zero-dep library